29 lines
862 B
Markdown
29 lines
862 B
Markdown
---
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title: partitionBy
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tags: array,object,function,advanced
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---
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Applies `fn` to each value in `arr`, splitting it each time `fn` returns a new value.
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- Use `Array.prototype.reduce()` with an accumulator object that will hold the resulting array and the last value returned from `fn`.
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- Use `Array.prototype.push()` to add each value in `arr` to the appropriate partition in the accumulator array.
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```js
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const partitionBy = (arr, fn) =>
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arr.reduce(
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({ res, last }, v, i, a) => {
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const next = fn(v, i, a);
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if (next !== last) res.push([v]);
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else res[res.length - 1].push(v);
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return { res, last: next };
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},
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{ res: [] }
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).res;
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```
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```js
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const numbers = [1, 1, 3, 3, 4, 5, 5, 5];
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partitionBy(numbers, n => n % 2 === 0); // [[1, 1, 3, 3], [4], [5, 5, 5]]
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partitionBy(numbers, n => n); // [[1, 1], [3, 3], [4], [5, 5, 5]]
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```
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